U.S. Pat. No. 4,005,330 to Homer H. Glascock, Jr. and John M. Anderson and U.S. Pat. No. 4,017,764, to John M. Anderson describe a class of induction ionized fluorescent lamps wherein a high frequency, solenoidal electric field is established by a transformer which is centrally disposed with respect to a substantially globular lamp envelope. The lamps described in those patents may be manufactured in a form which is electrically and mechanically compatible with the common screw base incandescent lamp and which provides substantially more efficient operation than conventional incandescent lamps.
The transformer which is utilized in the above-described fluorescent lamps generally comprises a primary winding coupled to an annular magnetic core, typically a ferrite, which is centrally disposed with respect to the lamp envelope and coupled to a fill gas therewithin. During lamp operation, power is transferred to a plasma in the fill gas which forms a single turn secondary linking the transformer core. The voltage drop around the plasma secondary is a function of the lamp geometry, core geometry, fill gas composition, and fill gas pressure. The peak magnetic flux within the transformer core is, in turn, a function of the voltage drop in the gas. The maximum voltage developed in the gas by such a transformer therefore, determines the saturation flux density of the core material.
U.S. Pat. Nos. 4,005,330 and 4,017,764 are incorporated in this specification as background material.
The voltage drop necessary to maintain operation of the above-described fluorescent lamps is typically less than 10 volts around the plasma secondary. It has been determined, however, that a potential of more than 400 volts is necessary to induce ionization and thus start a discharge in such lamps. Magnetic core structures which may be economically utilized for operating and maintaining a discharge in such lamps at a given frequency will generally not support sufficient magnetic flux levels to induce a 400 volt starting potential in the fill gas without saturating. Auxiliary means must, therefore, be provided to start a discharge by applying a high electric field to the gas within the envelope.
High starting voltages were, in the lamps of the prior art, generally developed by means of an additional transformer winding on the core. The additional winding, generally, was characterized by a high turns ratio with respect to the lamp primary and was thus able to generate much larger voltages, typically a thousand volts or more. Electrodes from the starting winding were coupled to the gas, typically through the lamp envelope. If the core was then excited to high flux levels, i.e., several times the running level, a small displacement current was coupled through the glass envelope and would tend to ionize the gas. The high flux level would cause the ionization to fill the envelope so that a running plasma condition was established.